1. Field of the Invention
The present invention relates to an image forming apparatus for maintaining a given image quality while suppressing degradation in graininess.
2. Description of the Related Art
Color stability of an output image is required for a color image forming apparatus which adopts an electrophotographic method. If each element of the apparatus varies due to use for many hours or a change in environment, however, the color of an image obtained by the color image forming apparatus also varies.
Japanese Patent Laid-Open No. 11-305515, therefore, proposes a technique of forming the pattern image of a solid image and a halftone pattern image for each color, and detecting the density of each pattern image by an optical sensor, thereby determining the development contrast. Note that the development contrast is an electric potential difference between an exposure electric potential formed on the photosensitive member of an image forming apparatus and a developing electric potential applied to the developing sleeve of a developing apparatus. An electric potential difference between a charging potential on the photosensitive member and the developing potential is referred to as back contrast.
The arrangement described in Japanese Patent Laid-Open No. 11-305515 can detect the width of a toner layer on an image carrier but cannot detect the thickness (height) of the toner layer. Even if the thicknesses of the toner layers of two pattern images are different from each other, therefore, the same density may be detected. In this case, wrong density control is executed, thereby lowering the output image quality.
When the image density is determined to be low, and the exposure light amount or the developing electric potential are controlled to increase the development contrast in order to enhance the image density, the toner amount carried on the photosensitive member increases. At this time, the toner layer on the photosensitive member increases not only in the surface direction of the photosensitive member but also in a direction (thickness direction) perpendicular to the surface. If the toner layer is too thick, toner spreads in the lateral direction when a toner image is transferred from the photosensitive member to an image carrier such as a printing medium or intermediate transfer member, and therefore, an area in which the toner image covers the image carrier becomes larger than a target. As the area in which the toner image covers the image carrier becomes large, the density visually becomes high or the image looks like an image in which the dot size is large, which means that the image quality drops. Furthermore, when forming an image by applying pressure in a transfer unit or fixing unit, a toner image readily spreads by the pressure if the height of the toner image is large, thereby degrading the graininess of the image. Note that the image quality is evaluated based on the graininess.
The graininess is, for example, an RMS graininess expressed by:
  graininess  =                    1        N            ⁢                        ∑                      i            =            1                    N                ⁢                              (                          Di              -                              D                _                                      )                    2                    where Di represents the density distribution, N represents the number of samples, and D represents the average density. Note that as the value of the RMS graininess is larger, the image quality degrades.